In this proposal, we plan to evaluate the potential for developing an antiviral chemotherapeutic strategy based on the intrinsic defense mechanism of human cells: mutagenesis of single-stranded DNA by cytidine deaminase APOBEC3G (A3G). Human immunodeficiency virus 1 (HIV-1) counters this activity by expressing virion infectivity factor (Vif) that downregulates both the activity and stability of A3G. The essential role of Vif in viral proliferation and highly regulated function of A3G make their interaction an attractive target for the resistance-free intervention. Furthermore, a possibility of generating dominant negative HIV-1 mutants in situ through A3G-induced mutagenesis presents a unique opportunity to inactivate HIV in latently infected cells, normally inaccessible to chemotherapeutics. The proposed research is based on the hypothesis that small molecules capable of competing with the Vif for A3G will simultaneously protect the cellular factor from the antagonism displayed by Vif and reduce stability of the viral factor. The challenge of identifying de novo inhibitors of protein-protein interaction will be addressed in this proposal by exploiting the unique power of genetic selection to yield rare solutions. We believe, therefore, that experimental validation of this hypothesis presents a unique opportunity for curbing the progression of HIV infections through a natural defense mechanism in latently infected cells without the danger of evolving resistant phenotypes. We plan to accomplish the following Specific Aims in developing this approach: Specific Aim 1: We will develop and implement a genetic selection scheme for the discovery of potent and selective antagonists of Vif-A3G interaction. Specific Aim 2: We will confirm and characterize Vif-degrading and/or A3G-protecting activities of the selected backbone cyclic peptides using a series of biochemical and cellular assays. Specific Aim 3: We will identify functional motifs important for the activity of the selected sequences and develop their cell-permeable derivatives. The expected outcome of this research will be a series of specific and potent cell-permeable agents capable of protecting A3G from the downregulation by Vif. Our long-term objective is to advance a prototype of a new class of anti-HIV drugs that can address the three existing problems associated with the current strategies: toxicity, resistance and latency. To protect organism against viral infections, human cells have developed a sophisticated defense mechanism involving modifications of viral genes, lethal to virus replication. Unfortunately, human immunodeficiency virus (HIV) is able to counteract this intrinsic protective system making the disease that it causes, Acquired Immunodeficiency Syndrome (AIDS), a global health problem, with no cure or prevention in sight. We propose to develop a novel anti-HIV therapeutic strategy that will rescue the natural antiviral mechanism without the danger of producing drug-resistant viruses.